1. Field of the Invention
The present invention relates to a circuit having a controlled impedance and more particularly to a circuit suitable for use at the input of an IF amplifier stage which is responsive to a gain control signal for varying the capacitance across a tuned bandpass filter coupled to the input of the IF amplifier stage for varying the frequency response thereof.
2. Description of the Prior Art
It is generally known that the bandpass characteristics of a tuned circuit can be varied by varying the reactance components comprising the same. Additionally, it is also known how to utilize tuned stages for providing bandpass characteristics to IF amplifier stages used in radio and television applications. For instance, see Buchsbaum, Fundamentals of Television (1964) pages 121-129 for a good description of a picture IF section of a typical television receiver. It has generally been established that the required video IF bandwidth of a television receiver should change with signal strength for optimum picture quality. Hence, with a strong input signal and a high signal to noise ratio, the bandwidth should be as wide as possible within the limitations of the incoming signal. Generally, the video carrier should be at -6 dB relative to the main bandpass to achieve correct transient performance for the video signal as is understood.
However, under low signal conditions, i.e., weak input signals with poor signal to noise ratio, a superior picture is obtained when the bandwidth of the video IF amplifier section of the television receiver is narrower and the picture carrier is only -1 or -2 dB relative to the main bandpass characteristics of the amplifier stage.
In the past this "blocking" of the IF bandpass circuit has been achieved by appropriate matching of filter elements of the IF amplifier stage to one or more discrete transistor amplifier elements which are under AGC control; i.e., the input or output impedance variation produced by gain control of the discrete transistor is used to move the filter pole/poles appropriately. A problem with this scheme is that the characteristic of the discrete transistor stage is subject to large variations between devices and so the characteristics of the bandpass modification is somewhat unpredictable.
Typically, integrated circuit structures such as the Motorola MC1349 "IF Amplifier" used for IF amplifier stages in a television receiver did not lend themselves easily to the aforedescribed treatment because they are designed for direct current coupling to following stages. Thus, large parameter changes are quite difficult to accommodate.
Thus, a need exists for a circuit to be utilized with an integrated IF amplifier circuit that can be used to vary the IF bandpass characteristics of the amplifier section to compensate for varying signal strengths while providing maximum picture quality in a television receiver.